1. Field of the Invention
The present invention relates to a control system, for a servo die cushion, which generates a force on a slide of a press machine using a servomotor as a drive source.
2. Description of the Related Art
It is known that a press machine, for press working such as bending, drawing or punching, can be provided with a die cushion mechanism, as an attached device, for applying a predetermined force or pressure, during the press working, to a movable support member (generally called a slide) supporting a first mold for press working, the force being generated by another movable member supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally called as a cushion pad) held at a predetermined pressure and, until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent the occurrence of wrinkles in a workpiece to be pressed by, for example, clamping an area around a site, of the workpiece to be pressed, between the cushion pad and the slide.
Many conventional die cushion mechanisms use a hydraulic or a pneumatic unit as a drive source. However, control by a hydraulic or a pneumatic unit may be carried out only under a constant pressure. It is preferable that the pressure during drawing is varied corresponding to the amount of the drawing. However, the pressure cannot be varied in a hydraulic or a pneumatic unit.
In recent years, therefore, a die cushion mechanism using a servomotor as a drive source has been used to carry out force control at a high speed, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall motions of the slide. The servomotor is activated by a predetermined force command corresponding to the position of the cushion pad and adjusts the force or pressure applied, to the slide, by the cushion pad while the cushion pad moves with the slide. The collision of, and pressure between, the slide and the cushion pad may be determined by detecting a load applied to the output of the servomotor via the cushion pad.
It could be said that one preferred control device for the die cushion is capable of suitably switching from the above force control to position control by using a position detector, or vice versa. For example, a control device as shown in FIG. 3 has a first speed commanding part 56, for generating a first speed command of the die cushion, based on a position command generated by a position commanding part 52 and a detected result by a position detector 54 for detecting the position of the die cushion. The control device 50 also has a second speed commanding part 62, for generating a second speed command of the die cushion, based on a force command generated by a force commanding part 58 and a detected result by a force detector 60 for detecting the pressure between the slide and the die cushion. The control device 50 further has a switch judging part 64 for judging that the command for controlling the die cushion should be switched from the first command to the second command, or vice versa, and a switching part 66 for switching the command based on the judgment of the judging part 64.
If the number of the die cushion to be controlled is one, the control device 50 may perform a control of the die cushion more preferable than only force control or speed control. However, when the cushion pad is relatively large and the cushion pad is supported by a plurality of die cushions, the control device 50 cannot perform suitable control. In other words, when a plurality of die cushions are used, each die cushion is independently controlled by an individual control device. Therefore, at a certain of time during press machining, there may be a situation in which one die cushion is controlled by position control and the other die cushion is controlled by force control. In such a situation, as the cushion pad supported by the plurality of die cushions is generally an integral member, an undesired mechanical distortion may occur in the cushion pad due to an excess force applied to the cushion pad. As a result, the accuracy of press machining may be reduced and the press machine may be damaged.